Semiconductor devices are developing rapidly due to the fast spread of information through media such as computers. Semiconductor devices are functionally required not only to operate with high speed, but also to have mass storage capacity. To meet these requirements, fabrication technology of semiconductor devices is developing to enhance integration, reliability, and response speed of semiconductor devices.
In case of Dynamic Random Access Memory (DRAM), 64 gigabyte (GB) DRAM with design rule of about 70 nanometers (nm) and 1 terabyte (TB) DRAM with design rule of about 35 nm will be developed by around the years of 2008 and 2014, respectively.
However, the 64 GB DRAM or 1 TB DRAM is not easy to fabricate using conventional methods such as optical lithography or chemical vapor deposition. Therefore, research for new fabrication methods are in progress.
A new technology using an electron beam or X-ray is being developed to replace the conventional photolithography technology. In addition, an atomic layer deposition method is being developed to replace the conventional methods for forming layers. Furthermore, research on semiconductor devices having quantum dots with sizes on the order of nanometers, which is applicable to a single-electron gate, are also in progress.
Examples of the semiconductor device with quantum dots and fabrication methods thereof are disclosed in U.S. Pat. No. 5,731,598 to Kado et al.; U.S. Pat. No. 6,060,743 to Sugiyama et al.; U.S. Pat. No. 6,090,666 to Ueda et al.; and U.S. Pat. No. 6,118,686 to Taira et al.
The fabrication method of quantum dots in development may be represented by a method using a Focused Ion Beam (FIB) or electron beam that forms a quantum dot by inserting more than one ion or one electron in a desired location using the FIB or electron beam. Using the FIB method, it is favorable to control the size and the location of formation of a quantum dot. One significant drawback to the FIB method is that the method has the limitation in commercial use because of low productivity.
U.S. Pat. No. 5,731,598 to Kado et al. discloses a single-electron tunnel device. To fabricate a single-electron device of a multiple junction structure operating at room temperature, metal and semiconductor clusters of several tens of nanometers are deposited to form causally quantum dots of several tens of nanometers. However, due to the casual formation of the multiple junction structure, size of a device cannot be reduced and efficiency of the fabrication is decreased consequently.
U.S. Pat. No. 6,090,666 to Ueda et al. discloses a method of forming crystal nuclei. In such a method, an amorphous thin layer is formed and a thermal processing is then performed for the thin layer to form a mono crystal. Subsequently, the mono crystal is formed as a quantum dot. The method is more favorable in view of productivity than the method taught by Kado et al., but has difficulty in controlling the size and the distribution of the quantum dots.
To fabricate a nano-device with a quantum dot, it is important to form the quantum dot with a crystal feature of single crystal level to form a fine and uniform quantum dot.
Consequently, a new method for forming a quantum dot, which can control the size and the distribution of the quantum dot easily and can also be employed commercially, is required.